1. Field of the Invention
The present invention relates to corrugating adhesives which contain solubilized plant fiber, such as corn fiber, and polyvinyl alcohol. More particularly, the invention relates to high speed corrugating adhesives of the carrier, no-carrier or carrier-no-carrier type which are prepared by incorporating solubilized corn fiber in the adhesive formulation to obtain excellent pin adhesion and improved water resistance and incorporating polyvinyl alcohol to obtain improved cross-bonding functionality and higher line speeds.
2. Description of Related Art
Adhesives used in manufacturing corrugated board are usually comprised of an aqueous emulsion of starch, caustic, a boron containing compound and, where water resistance is needed, a waterproofing resin. The main binder of corrugating paper is the starch which is gelatinized in the corrugating process as it penetrates the paper fiber. The other components, namely, the water, caustic, boron containing compound, and waterproofing resin are auxiliary agents which modify the basic properties of the starch.
Caustic, usually in the form of sodium hydroxide, directly affects the gelatinization point temperature of the starch. Gelatinization point temperature is often referred to as gel point or gel temperature and is reduced from its natural value of 165.degree. F. (74.degree. C.) for unmodified corn starch to a temperature in the range from about 138.degree. F. (59.degree. C.) to about 160.degree. F. (71.degree. C.) by the addition of caustic. Caustic also enhances penetration of the gelatinized starch into the surface paper fibers allowing for better bonding.
The boron containing compounds perform multiple tasks. The most important function is developing the adhesive tackiness that is crucial in the formation of the "green bond". The green bond is the bond which holds the components of the corrugated board together until final heat curing of the adhesive system. It relies on the viscosity and the tack of the adhesive to maintain the integrity of the product during curing. Corrugating machine speeds therefore are limited by the rate of viscosity increase (i.e., rate of green bond formation) in the bond line between the liner and the corrugated medium. The oxygen atoms attached to the boron form stronger bonds, sometimes called boron bridges, between the starch and cellulose hydroxyl groups. Boron containing compounds also act as buffering agents in the presence of caustic and help maintain the viscosity stability of the adhesive paste.
It is known that polyhydroxy compounds other than starch, such as polyvinyl alcohol, will respond with the boron containing compounds in a manner similar to that with starch. Boron containing compounds and polyvinyl alcohol are believed to interact synergistically to form strong bonds. Like other polyhydroxy compounds, polyvinyl alcohol in the presence of starch will develop adhesive tackiness faster in the presence of boron, which means that the corrugator can operate at higher machine speeds.
Water proof or water resistant resins are considered to be an optional component of adhesives, and although some corrugating plants may not need them, most plants are prepared to use them when needed. Many types of thermo-setting resins are suitable. The most common of them are derived from urea-formaldehyde, ketone-formaldehyde or melamine-formaldehyde. When heat and pressure is applied to them in a corrugating machine, they will condense to form an excellent water resistant network. When polyvinyl alcohol is added to adhesives containing these resins, it participates in cross-linking which will improve the water resistant bonds.
In the corrugating process, adhesive is commonly applied to the tips of the flutes of a corrugated medium. Then a noncorrugated flat paper liner is applied against the adhesive coated flutes as they pass between a corrugating roll and a pressure roll. The resulting product has the corrugating medium on one side and a flat liner on the other side and is called a single-faced portion. The single-faced portion may be used "as is" (called a "single facer" board) or adhesive may be applied to the flute tips of the single-faced portion and a second flat sheet can be applied in the same manner as the first in what is called a "double-facer" or a "double-backer" operation. The second liner sheet is treated with heat and reduced pressure (relative to the pressure used to make a single-faced portion) immediately following contact with the adhesive.
Starch-based adhesives which can be of the carrier, no-carrier and carrier-no-carrier type are commonly used in processes for manufacturing corrugated paper board. In carrier type adhesives, a portion of the starch (or dextrin) forms a carrier, often known as the gelatinized phase, which suspends the balance of the starch which is in an ungelatinized state. Under conditions of heat and pressure, the ungelatinized starch is rapidly hydrated and gelatinized to increase quickly the viscosity and adhesivity of the adhesive composition. In no-carrier type adhesives, all of the starch is slightly cooked or swollen with heat and caustic soda for viscosity. Finally, carrier-no-carrier type adhesives have a portion of the starch which forms a carrier and is responsible for about one half of the viscosity and the remaining viscosity is obtained by slightly swelling the uncooked starch.
One of the most important properties of a corrugating adhesive is green strength. Green strength is the characteristic that holds the paper together until the full strength of an adhesive develops. The green strength contributed by conventional starch adhesives is fairly low, but the addition of substances like polyvinyl alcohol improve the property as noted above. Increased levels of green strength, sometimes called early tack, allow corrugating machine operators to increase the speed of their machines without sacrificing the quality of the product.
Polyvinyl alcohol has been added to starch based corrugating adhesives for many years to increase the viscosity and the strength of the bond between the corrugating medium and the liner material. In Japanese Patent Publication Number 45-19600 to Imoto et al. (Jul. 4, 1970), for example, a suspension of powdered polyvinyl alcohol and ungelatinized starch in an alkaline solution of gelatinized starch and borax is described for use in producing highly water-resistant corrugated board. Imoto et al. discloses the use of totally saponified (i.e. fully hydrolyzed) polyvinyl alcohol and water-soluble derivates thereof.
In U.S. Pat. No. 4,094,718 to Czerwin, a fully hydrolyzed polyvinyl alcohol having a fine particle size and low cold water solubles is used in a modified starch-based corrugating adhesive containing borax, alkali and water. The polyvinyl alcohol is in suspension in an aqueous emulsion of the adhesive and is dissolved during the corrugating process when the paper passes between the heated rolls of a corrugating machine.
U.S. Pat. Nos. 4,600,739 and 4,677,145 to Krankkala disclose corrugating adhesives comprised of an aqueous suspension of unmodified starch, caustic, a boric acid compound and a water soluble, partially hydrolyzed, polyvinyl alcohol. Based on the specifications of both patents, these adhesive compositions allow corrugators to attain high line speeds with improved green bond strength. According to U.S. Pat. No. 4,826,719 to Murdock et al., however, it was found that the amounts of soluble polyvinyl alcohol that could be used under the two prior Krankkala patents was limited by the fact that at higher concentrations than claimed, the soluble polyvinyl alcohol tended to form gels as was recognized in Czerwin and Imoto et al.
The Murdock et al. patent discloses a corrugating adhesive based on unmodified starch and containing an alkaline metal hydroxide, a boric acid compound and a cold water insoluble, fully hydrolyzed, polyvinyl alcohol which is heat solubilized in the adhesive composition before application. According to the patent specification, this adhesive composition also allows corrugators to attain high line speeds.
U.S. Pat. No. 5,093,393 to Faber et al. describes the use of an intermediately hydrolyzed polyvinyl alcohol (92-98% hydrolyzed) in starch-based corrugating adhesives, and U.S. Pat. No. 5,075,360 to Fitt et al. discloses the in situ hydrolysis of a cold water soluble polyvinyl alcohol to make a corrugating adhesive having polyvinyl alcohol which is more than about 95% hydrolyzed. In each case, polyvinyl alcohol is said to improve the starch-based corrugating adhesives to which it is added.
Synthetic polymers that affect green strength tend to be expensive and their prices follow the volatility of the petroleum market. These polymers also are perceived by the public as less desirable than natural products. Moreover, the use of such polymers can require additional processing steps which adds expense for the corrugator.
Another important property of corrugating adhesives is known as dry pin adhesion. This is a measure of bond strength and the percentage of paper failure versus adhesive failure.
According to U.S. Pat. No. 4,941,922, fibers can be added to starch-based corrugating adhesives to enhance adhesion and dispersion and yield improved adhesive characteristics including increased waterproofness, dry strength, viscosity and adhesiveness. The fibers disclosed generally are insoluble fibers which have certain mechanical properties in that they provide interlocking filaments. Suitable sources of fiber are described as cellulosic fiber including wood, paper, cotton and rayon; fibers of synthetic origin including Nylon, polyester, polypropylene, Lycra Spandex, Vyrene, Vinyon, Dynel, Saran, Creslan, acrylic, polyethylene, Teflon, tetrafluoroethylene, glass, Corlon and metallic fibers. Fibers generated during the wheat or corn milling processes are also said to be suitable. According to the patent, the fibers typically are suspended initially in the uncooked starch portion, suspended with the carrier portion, suspended prior to the carrier-no-carrier (or no-carrier) swell or added to the final adhesive mixture prior to storage or use. There is no experimental data in the patent, however, describing the use of fibers generated during the corn milling process and the patent does not disclose the extraction of hemicellulose from fibers of any kind. Furthermore, the fibers must be present in an amount from about 2.2 to about 22 pounds of fiber per 100 pounds of corrugating adhesive and they must be large, having a diameter of between about 0.0005 inches and about 0.02 inches and a length of between about 0.025 inches and 0.25 inches.
It has been found that solubilized plant fiber, particularly corn fiber, a readily available component of corn kernels and hulls, and other plant materials, can be used to enhance dry pin adhesion in starch-based corrugating adhesives. The use of solubilized plant fibers also provides excellent green strength, particularly at low viscosities, which allows for a wider range of viscosities compared with adhesives based primarily on starch. The solubilized fiber can replace some or all of the added starch in typical adhesives.
It has also been found that flatter board can be produced with adhesives containing solubilized plant fibers. This is advantageous when the board is cut and printed. The board also has less fractured liners and cracked scores compared with board produced using adhesives without plant fiber.
Applicants now have discovered that adding polyvinyl alcohol to adhesives containing solubilized plant fiber can further improve line speeds. It is a theory of the invention that the interaction of polyvinyl alcohol with solubilized plant fiber is similar to the interaction of polyvinyl alcohol with starch in the enhancement of cross-bonding functionality on dehydration, particularly in the presence of boron compounds or condensation polymers during the production of waterproofing adhesive.
In the present specification and claims, all parts and percentages are by weight unless otherwise specified, and the term "parts of adhesive" is used to refer to the total weight of adhesive, including water, starch and all chemicals. The term "parts of carrier phase" is used to refer to the total weight of the carrier phase, including water, starch and all chemicals. The terms "total starch" and "total carbohydrate" refer to the total cumulative weight of unmodified starch, modified starch and dextrin present, i.e. all of the carbohydrates. The term polyvinyl alcohol is sometimes written as "poly(vinyl alcohol)" or abbreviated as "PVOH".